The present invention relates to a method and a device for imaging an object using a two-dimensional array of ultrasonic transducer elements. Such a method and such a device are discussed in German Patent No. DE-28 41 694 C3 and the corresponding U.S. Pat. No. 4,219,846, for example.
In ultrasound diagnostic medicine and during nondestructive ultrasonic testing of workpieces, a volumetric area of the object to be imaged is irradiated by ultrasonic pulses in a transmit mode. A signal-processing unit in a main unit receives reflected ultrasonic echo pulses, constructs an ultrasonic image corresponding to a two-dimensional (2-D) cross-section through the object based on the received echo pulses, and displays the constructed ultrasonic image on a monitor.
Under the state of the art, one-dimensional (1-D) phased arrays transmit and receive ultrasonic pulses. These arrays include piezoelectric transducer elements driven by an electronic control unit with selected time delays (often described as phase lags). Such arrays, driven with time delay, permit steered (or "swiveled" or "pivoted") ultrasonic beams to be transmitted and received and permit the ultrasonic beams to be focused in a plane which is fixed relative to the normal to the array surface and which is fixed relative to the longitudinal direction of the array.
Generally, the smaller the transducer elements the larger the steering angle, measured relative to the normal, can be for the ultrasonic beam. The spacing of the transducer elements is generally selected to be approximately half the wavelength of the ultrasound to avoid additional diffraction patterns (i.e., side lobes). For example, at a diagnostic test frequency of 3.5 MHz, the spacing of the transducer elements is about 0.2 mm. On the other hand, the array must have a defined minimum length to achieve an adequate acoustic amplitude and to exactly focus the beam. From these two requirements, namely from the maximum spacing of the transducer elements and the minimum length of the array, a minimum number (typically 64) of transducer elements for the array can be determined.
In addition to one-dimensional ultrasonic arrays, prototypes of two-dimensional (2-D) transducer arrays are also known. Such two-dimensional transducer arrays are generally rectangular, circular or annular in shape. A rectangular two-dimensional array is usually constructed as an M.times.N-matrix of individual, generally rectangular, transducer elements. If these transducer elements are driven with properly selected time delays, then in contrast to the one-dimensional arrays in the transmit mode, an ultrasonic beam can be generated that can be steered and, in general, focused not only in one, but in two angular directions.
To scan a large enough solid-angle area with the ultrasonic beam, the conditions of a maximum spacing of typically about 0.2 mm between the transducer elements and of a minimum surface (aperture) of the 2-D array of typically about 20.times.20 mm.sup.2 should be met for a quadratic array (i.e., an array in which N=M) and 3.5 MHz diagnostic frequency. That is, the maximum spacing requirement and minimum surface requirement of the two-dimensional array is analogous to the requirements for the one-dimensional arrays discussed above. Consequently, a minimum number of transducer elements is also required for the two-dimensional array, which can amount, for example, to 64.times.64=4096.
Such a large number of transducer elements and the requisite small dimensions entail problems for manufacturing and contacting (i.e., connecting) the transducer elements. The large number of control or data lines required to transmit the control signals and the video signals, as well as the parasitic capacitances on the data lines, also present problems.
In one known way for alleviating these problems, the number of transducer elements in the array is reduced in accordance with a geometric specification or statistically (See Turnbull et al., "Beam Steering with Pulsed Two-Dimensional Transducer Arrays," IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Vol. 38, No. 4, pp. 320-333 (July 1991); See also, Smith et al., "High-Speed Ultrasound Volumetric Imaging System--Part I: Transducer Design and Beam Steering," IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Vol. 38, No. 2, pp. 100-108 (March 1991); and van Ramm et al., "High Speed Ultrasound Volumetric Imaging System--Part II: Parallel Processing and Image Display," IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Vol. 38, No. 2, pp. 109-115 (March 1991)). However, when these measures are used, the beam characteristic of the 2-D array will be adversely affected.
The German Patent Publication No. DE-36 90 124 T1, which corresponds to U.S. Pat. No. 4,608,868, discusses an ultrasonic imaging device including a curved transducer for producing an ultrasonic beam focused on a focusing volume.
The German Patent No. DE-30 19 409 A1, which corresponds to U.S. Pat. No. 4,412,316 ("the '316 patent"), discusses an ultrasonic transducer matrix in which the transducer elements on the two opposite facing flat sides of the matrix are different, but are interconnected in a definitively prescribed manner. 0n a first flat side of the matrix, parallel rows of transducer elements are each contacted by a shared control line. On the second flat side of the matrix, facing opposite to the first flat side, a transducer element in the center of the matrix, and groups of transducer elements that are arranged concentrically with respect to this transducer element in the center of the matrix, are each contacted, via a shared concentric electrode, by one shared control line. The concentric groups of transducer elements can be square, annular, or star-shaped.
In one embodiment of the '316 patent, the shared electrodes for the concentric groups of transducer elements are each connected to zero potential (ground). A time-delayed transmit pulse, having the same delay for all matrix rows, is applied to each of the matrix rows of transducer elements via the control electrodes on the first flat side of the matrix. An ultrasonic beam, which can be steered within a solid-angle area, is produced since all matrix rows are driven with a linear delay. The steering angle depends on the magnitude of the linear delay. By having a quadratic delay of the transmit pulses for the matrix rows overlap the linear delay, an ultrasonic beam that can be steered, as well as focused, is produced.
In another embodiment of the '316 patent for producing a focused ultrasonic beam, the rows of the transducer elements on the first flat side of the matrix are connected to zero potential (ground), and each of the concentric groups of transducer elements on the second flat side of the matrix are driven, via the respectively allocated shared electrode, with the same quadratic delay in the transmit mode, and are read out in the receive mode. The depth of focus of the ultrasonic beam can be controlled by varying the quadratic delay. In this second variant, steering the ultrasonic beam is not possible.
U.S. Pat. No. 5,186,175 discusses another ultrasonic imaging device having an ultrasonic transducer matrix. The transducer matrix can be electrically connected via, a switching circuit, to both transmitter and receiver electronics. The switching circuit interconnects the transducer elements of the matrix by rows or columns. These rows or columns of transducer elements are driven jointly in the transmit mode and are read out jointly in the receive mode. In this manner, the number of signal lines can be reduced to the number of transducer elements in each row or column. A special evaluation with delay elements in the receive mode (beam forming) enables the received signals to be assigned to a preselected focal point in the object being diagnosed. This known ultrasonic imaging device does not permit the ultrasonic beam to be steered within a solid angle and does not permit a dynamic focusing in two spatial directions.
German Patent No. DE-28 41 694 C3, which corresponds to U.S. Pat. No. 4,219,846, discusses an ultrasonic imaging system having a rectangular, matrix-shaped transducer array. In a selected, rectangular scanning zone, which includes a portion of the transducer elements of the array, a portion of the transducer elements are interconnected by a switch circuit to form a selected configuration of sub-arrays.
The sub-arrays are at least approximately arranged in the shape of concentric rings around a central sub-array in the center of the scanning zone. The transducer elements of each sub-array are jointly driven in the transmit mode with a selected time delay via a shared control line for the sub-array, and are jointly read out in a following receive mode via a shared signal line for the sub-array. By interconnecting the transducer elements in the scanning zone into concentric rings, an ultrasonic beam, which is propagated parallel to the surface normal of the matrix, and which is focused in two spatial directions, is produced.
The focusing range of the ultrasonic beam forms one pixel of the ultrasonic image. The scanning zone is now moved along the longitudinal axis of the rectangular transducer matrix. The transducer elements of the displaced scanning zone are interconnected to allow the same configuration of the concentric sub-array to be formed again with the same focusing properties. Thus, by longitudinally shifting the scanning zone across the matrix, the object is linearly scanned, whereby each pixel of the image corresponds to the part of the object situated in the focusing range of the particular scanning zone. In so doing, the ultrasonic beam is shifted along the longitudinal axis of the matrix and always remains parallel to the surface normals of the matrix and is adjusted to a fixed focusing spacing. In the case of this known system, the ultrasonic beam cannot be steered within a predetermined solid angle.
The object of the present invention is to provide a method and a device to ultrasonically image an object with a two-dimensional array, which will enable an ultrasonic beam to be produced that can be steered within a selected solid angle and, which, at the same time, will clearly reduce the number of different time delays required in the transmit mode and which will reduce the influence of parasitic capacitances on the received signals in the receive mode.